1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for transmitting a midamble in a wireless communication system.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4th generation) mobile communication standard following the IMT-2000. It was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
Effective transmission/reception methods and utilizations have been proposed for a broadband wireless communication system to maximize efficiency of radio resources. An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading, and an interval of a transmitted symbol is increased, thereby minimizing inter-symbol interference.
When a system uses the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the plurality of users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner. In an OFDMA system, frequency diversity for multiple users can be obtained by using frequency selective scheduling, and subcarriers can be allocated variously according to a permutation rule for the subcarriers. In addition, a spatial multiplexing scheme using multiple antennas can be used to increase efficiency of a spatial domain.
MIMO technology can be used to improve the efficiency of data transmission and reception using multiple transmission antennas and multiple reception antennas. MIMO technology includes spatial multiplexing, transmit diversity, beam-forming and the like. An MIMO channel matrix according to the number of reception antennas and the number of transmission antennas can be decomposed into a number of independent channels. Each of the independent channels is called a layer or stream. The number of layers is called a rank.
A pilot can be transmitted from a base station (BS) to a user equipment (UE) through a downlink. The pilot can also be referred to as other terminologies such as a reference signal according to a wireless communication system. The pilot may be used to perform channel estimation or to measure a channel quality indicator (CQI). The CQI may include a signal to interference plus noise ratio (SINR), a frequency offset estimation, etc. To optimize system performance in different transmission environments, an 802.16m system provides a common pilot structure and a dedicated pilot structure. The common pilot structure and the dedicated pilot structure can be identified according to a resource in use. A common pilot can be used by all UEs. A dedicated pilot can be used by a UE to which a specific resource is allocated. Therefore, precoding or beamforming may be performed on the dedicated pilot in the same method as that used for a data subcarrier. A pilot structure can be defined for up to 8 transmission streams, and may have a unified pilot structure according to the common pilot and the dedicated pilot.
A midamble is a signal transmitted by a BS to allow a UE to directly measure a channel state. When the BS transmits signals by using a multiple input multiple output (MIMO) technique through a plurality of antennas, the BS can transmit different signals for the respective antennas or transmit signals at different locations in a resource region, and the UE can measure a channel state for each antenna of the BS by receiving a midamble and thus estimate a channel state of a serving cell or an interference level of a neighbor cell. The BS can adaptively schedule resources by receiving a feedback of the channel state estimated by the UE.
When transmitting the midamble for each antenna, a frequency partition or a reuse factor has to be considered. In this case, the midamble transmitted through each antenna can be multiplexed in various manners. If the number of transmit antennas is different between neighbor cells, a level of interference influenced by the neighbor cell may differ according to a location of a subcarrier to which a midamble sequence is mapped. In addition, the midamble may have a similar structure as that of a preamble, and thus the UE may erroneously receive a preamble instead of a detected midamble. Accordingly, there is a need for a robust midamble structure by which the UE can correctly detect a midamble irrespective of the number of transmit antennas.